Light emitting diode driving circuit and light emitting diode array device

ABSTRACT

There is provided an LED driving circuit including: at least one ladder network circuit including: (n+1) number of first branches connected in parallel with one another by n number of first middle junction points between a first junction point and a second junction point, where n denotes an integer satisfying n≧2, (n+1) number of second branches connected in parallel with one another by n number of second middle junction points between the first junction point and the second junction point, the (n+1) number of second branches connected in parallel with the first branches; and n number of middle branches connecting the first and second middle junction points of an identical m sequence to each other, respectively, wherein each of the first and second, and middle branches comprises at least one LED device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/213,506, filed on Jun. 20, 2008 now U.S. Pat. No. 8,026,675, whichclaims the priority of Korean Patent Application No. 2007-61593 filed onJun. 22, 2007, in the Korean Intellectual Property Office, thedisclosures of each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting diode (LED) circuit,and more particularly, to an LED driving circuit directly applicable toan alternating current (AC) power without requiring a converter forconverting the AC into a direct current (DC), and an LED arrayapparatus.

2. Description of the Related Art

A semiconductor light emitting diode (LED) is advantageous as a lightsource in terms of output, efficiency or reliability. Thus, the LED isactively researched and developed as a high-output and high efficiencylight source to substitute a backlight of a lighting device or a displaydevice.

In general, the LED is driven in a low driving current (DC). Therefore,to be driven at a normal voltage, i.e. alternating current (AC) of 220V,the LED requires an additional circuit, e.g., AC/DC converter forsupplying a low DC output voltage. However, such an additional circuitcomplicates configuration of an LED module and furthermore potentiallyundermines efficiency and reliability when a supply power source isconverted. Moreover, the additional part other than the light sourceincreases costs and size of the product and also degradeselectromagnetic interference (EMI) characteristics due to a periodiccomponent when operating in the switching mode.

To overcome this problem, various types of LED driving circuits capableof being driven without an additional converter have been suggested.However, in a conventional AC-driven LED driving circuit, most LEDs arearranged to be driven in a specific half cycle of the AC voltage,thereby increasing the number of LEDs required for achieving desiredlight amount.

The necessary number of LEDs may be varied according to arrangement ofthe LEDs even though identical light amount is supplied. But theconventional arrangement of LEDs ensures very low efficiency. Forexample, in a conventional representative example where the LEDs arearranged in a reverse parallel or bridge configuration, the actualnumbers of LEDs continuously emitted represent merely 50% and 60% oftotal numbers of LEDs, respectively. That is, a greater number of LEDsare inefficiently required to attain desired emission.

Therefore, the LEDs may be more efficiently arranged to assure identicallight amount through a smaller number of LEDs. This arrangement withgreater efficiency is of significant importance to assure costefficiency of the AC-driven LED circuit.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a novel light emitting diode(LED) driving circuit which requires a minimum number of LEDs whiledirectly employing an alternate current (AC) voltage.

An aspect of the present invention also provides an LED array apparatusincluding the LED driving circuit.

According to an aspect of the present invention, there is provided anLED driving circuit including: at least one ladder network circuitincluding: (n+1) number of first branches connected in parallel with oneanother by n number of first middle junction points between a firstjunction point and a second junction point, where n denotes an integersatisfying n≧2, (n+1) number of second branches connected in parallelwith one another by n number of second middle junction points betweenthe first junction point and the second junction point, the (n+1) numberof second branches connected in parallel with the first branches; and nnumber of middle branches connecting the first and second middlejunction points of an identical m sequence to each other, respectively,wherein each of the first and second, and middle branches includes atleast one LED device, and m is a positive integer defining sequences ofthe respective (n+1) first and second branches and the n number ofmiddle branches with respect to the first junction point; a firstcurrent loop having a first group of LED devices located on (2m−1)sequence of the first branches, 2m sequence of the second branches andthe n number of middle branches, respectively to be connected in serieswith one another and driven in a first half cycle of an alternatingvoltage applied between the first and second junction points; and asecond current loop having a second group of LED devices located on 2msequence of the first branches, (2m−1) sequence of the second branchesand the n number of middle branches, respectively to be connected inseries with one another and driven in a second half cycle of analternating voltage between the first and second junction points.

The each of the first and second branches and middle branches mayinclude one LED. Alternatively, at least one of the first, second andmiddle branches may include a plurality of LEDs connected in series toone another.

The first and second braches may include one LED, respectively, and themiddle branches may include a plurality of LEDs connected in series toone another. Considering a reverse directional voltage, the middlebranches may include one of two and three LEDs connected in seriestogether.

The ladder network circuit may include a plurality of ladder networkcircuits connected in series to one another, wherein a second junctionpoint of one of the ladder network circuits is in contact with a firstjunction point of another one of the ladder network circuits.

According to another aspect of the present invention, there is providedan LED array apparatus including a plurality of LEDs, the LED arrayhaving the LED driving circuit defined as described above.

According to still another aspect of the present invention, there isprovided an LED array apparatus including: K number of first LEDsconnected in a row by n number of first middle junction points betweenfirst and second junction points, each of the first middle junctionpoints having electrodes of identical polarity connected thereto, wheren is an integer satisfying n≧2 and K is an integer satisfying K≧3,wherein the electrodes connected to the first middle junction pointshave polarities arranged alternately from the first junction point,starting with a first polarity; L number of second LEDs connected in arow by n number of second middle junction points between the first andsecond junction points, each of the second middle junction points havingelectrodes of identical polarity connected thereto, where L is aninteger satisfying L≧3, wherein the electrodes connected to the secondmiddle junction points have polarities arranged alternately from thefirst junction point, starting with a second polarity; and M number ofthird LEDs connected between the first and second middle junction pointseach having an identical sequence from the first junction point, each ofthe third LEDs having an electrode connected to have an oppositepolarity to a corresponding one of the electrodes of the first andsecond LEDs, where m is a positive integer defining sequence of the nnumber of first and second middle junction points with respect to thefirst point junction, and M is an integer satisfying M≧n.

The first and second LEDs may include (n+1) number of first and secondemitting diodes, respectively, the first and second LEDs connected toidentical polarity, respectively.

Each of the third light emitting diodes may be connected between thefirst and second middle junction points, respectively.

The plurality of third LEDs may be connected between the first andsecond middle junction points, respectively, and the third LEDs betweenthe first and second middle junction points may be connected in one ofseries and parallel to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a light emitting diode (LED) driving circuitaccording to an exemplary embodiment of the invention;

FIGS. 2A to 2C illustrate LED driving circuits according to anotherexemplary embodiment of the invention;

FIG. 3 illustrates an LED driving circuit according to still anotherexemplary embodiment of the invention;

FIGS. 4A to 4C illustrate LED driving circuits according to aconventional example and according to an exemplary embodiment of theinvention; and

FIGS. 5A to 5B illustrate LED driving circuits according to anotherconventional example and according to another exemplary embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

FIG. 1 illustrates a light emitting diode (LED) driving circuitaccording to an exemplary embodiment of the invention.

The LED driving circuit includes a ladder network LED circuit. That is,the ladder network LED circuit of the present embodiment includes threefirst branches connected to one another by first middle junction pointsc1 and c2 between first and second junction points a and b, and threesecond branches connected to one another by second middle junctionpoints d1 and d2 between the first and second junction points a and b.The LED driving circuit includes two middle branches connected betweenthe first and second middle junction points c1, d1, c2 and d2 of anidentical sequence. Here, LED devices 8, 9, 10, 11,12,13,14, and 15 aredisposed on the first and second branches and the middle branches,respectively.

The LED driving circuit includes two current loops L1 and L2 to bedriven in different half cycles of an alternating voltage. The firstcurrent loop L1 includes the LED devices 8, 9, 10, 11, and 12 connectedin series to one another to be driven in a first half cycle of thealternating voltage. The second current loop L2 includes the LED devices13, 11,14,9, and 15 connected in series to one another to be driven in asecond half cycle of the alternating voltage. As described above, withthe alternating voltage applied, the circuit can be operated such thatthe LED devices 9 and 11 are driven in a bi-direction.

The LEDs are arranged in the ladder network circuit as described belowwhen the first and second branches and middle branches from the firstjunction point a have a sequence defined as m.

The LED devices 8, 9, 10, 11,12,13,14, and 15 may be divided into firstand second LED groups according to a cycle of the drivable alternatingcurrent. The first LED group includes the LEDs 8, 9, 10, 11, and 12belonging to the first branch of an odd number of (2m−1) sequence, allmiddle branches and the second branch of an even number of 2m sequence.The LEDs 8, 9, 10, 11, and 12 of the first LED group are connected inseries to one another. The second LED group includes the LEDs13,11,14,9, and 15 belonging to the first branch of an even number of 2msequence, all middle branches and the second branch of an odd number of(2m−1) sequence. The second LED group is connected in series to oneanother to be reverse in polarity to the first LED group.

Therefore, the first LED group may form the first current loop L1 drivenin the first half cycle of the alternating voltage and the second LEDgroup may form the second current loop L2 driven in the second halfcycle of the alternating voltage. In this driving configuration, the LEDdevices 9 and 11 located on the middle branches and commonly belongingto the first and second LED groups can be operated continuously over anentire cycle of the alternating voltage.

As described above, in the LED driving circuit including the eight LEDdevices 8, 9, 10, 11,12,13,14, and 15, the two LED devices 10 and 14 canbe driven over the entire cycle of the alternating voltage. Therefore,in the actual ladder network, five LED devices may be provided toperform continuous emission. Here, a ratio of the number of driven LEDsto the number of employed LEDs is 62.5%. This figure is higher than inthe case of the conventional AC-driven LED arrangement, for example,reverse polarity arrangement (50%) or bridge arrangement (60%).

The LED driving circuit of the present embodiment is significantlydifferent from a bridge structure since the LED device 9 and the LEDdevice 11 are connected to each other in series, not in parallel. Thatis, in the LED driving circuit of the present embodiment, the LEDdevices 10 and 14 are inserted to be connected in series with each otherbetween the LED device 9 and the LED device 11. This produces a laddernetwork structure fundamentally different from the bridge structure.

In the LED driving circuit of the present embodiment, the LEDs driven ina bi-direction, i.e., over an entire cycle of the alternating voltage,are connected in series, not in parallel by inserting the LED devices 10and 14 and connecting four middle junction points c1, c2, d1, and d2.This LED arrangement structurally forms one loop. In actual driving, asdescribed above, the LEDs are different in potential difference in theloop formed of the middle junction points and thus operated in oneserial configuration without forming a current loop.

According to another exemplary embodiment of the invention, in theladder network structure shown in FIG. 1, a loop connecting the firstand second middle junction points together may be provided as one stackand various LED driving circuits may be successively connected to oneanother by the plurality of stacks. That is, the first and second middlejunction points may be configured in identical numbers of at leastthree, respectively. The first and second branches may be configured inidentical numbers of four, respectively.

FIG. 2A illustrates an LED driving circuit having four first and secondmiddle junction points c1,c2,c3,c4 and d1,d2,d3,d4, respectivelyaccording to another exemplary embodiment of the invention.

The LED driving circuit shown in FIG. 2A includes four middle branchesconnecting the first and second middle junction points of an identicalsequence. This driving circuit may be understood as a ladder networkcircuit having three stacks.

Referring to FIG. 2A, each of the branches has one LED device disposedthereon. These LED devices are arranged to include first and secondcurrent loops driven in different half cycles of an alternating voltage.That is, in a first half cycle of the alternating voltage, correspondingones of the LED devices are connected in series to one another to formthe first current loop along A1-C1-B2-C2-A3-C3-B4-C4-A5. In a secondhalf cycle of the alternating voltage, other corresponding ones of theLED devices are connected in series to one another to form the secondcurrent loop along B1-C1-A2-C2-B3-C3-A4-C4-B5.

In the LED driving circuit of the present embodiment, the four LEDdevices C1, C2, C3, and C4 located on the middle branches and commonlyinvolved in the first and second current loops can be operatedcontinuously over an entire cycle of the alternating voltage.

As described above, in the LED driving circuit including a total offourteen LED devices, the four LED devices C1, C2, C3, and C4 can bedriven over an entire cycle of the alternating voltage. This allows thenine LED devices to continuously emit light in the actual ladder networkcircuit. Here, the LED utilization efficiency is about 64%. In thepresent embodiment, a smaller number of LEDs can be utilized than in theprevious embodiment.

In the driving circuit shown in FIGS. 1 and 2A, each of the first andsecond branches and middle branches is illustrated to include one LEDdevice. Alternatively, the each of the first and second branches andmiddle branches may include the plurality of LED devices. However, evenin this case, the plurality of LED devices belonging to the identicalbranch should be connected in series to one another.

Particularly, an increase in the number of the LEDs in the middlebranches leads to a relative increase in the number of the LEDs drivenin a bi-direction. This accordingly enhances light emitting efficiencyfor the LEDs employed. Consequently, this diminishes the number of theLEDs required for obtaining desired emission level.

The LED driving circuit shown in FIG. 2B is configured such that two LEDdevices are connected in series to each other on each of the middlebranches in the LED driving circuit of FIG. 2A.

In the first half cycle of the alternating voltage, corresponding onesof the LED devices are connected in series to one another to form thefirst loop along A1-C1-C1′-B2-C2-C2′-A3-C3-C3′-B4-C4-C4′-A5. In thesecond half cycle of the alternating voltage, other corresponding onesof the LED devices are connected in series to one another to form thesecond current loop along B1-C1-C1′-A2-C2-C2′-B3-C3-C3′-A4-C4-C4′-B5.

In the LED driving circuit of the present embodiment, eight LED devicesC1, C1′, C2, C2′, C3, C3′, C4, C4′ belong to the middle branches. Thatis, two times greater number of LED devices C1,C1′,C2,C2′,C3,C3′,C4, andC4′ are commonly involved in the first and second current loops toensure continuous operation over an entire cycle of the alternatingvoltage compared to the LED driving circuit of FIG. 2A.

In consequence, in the LED driving circuit including a total of eighteenLED devices, the eight LED devices C1, C1′, C2, C2′, C3, C3′, C4, andC4′ can be driven over an entire cycle of the alternating voltage.Therefore, thirteen LED devices may continuously emit light in theactual ladder network circuit. Here, the utilization efficiency of theLEDs is about 72%. In the present embodiment, a much smaller number ofLEDs are employed than in the previous embodiments.

An LED driving circuit shown in FIG. 2C is configured such that in theLED driving circuit shown in FIG. 2A, LED devices A1′, B2′, and C3′ aredisposed on a first branch of first sequence, a second branch of secondsequence and a middle branch of third sequence, respectively to beconnected in parallel to each other.

In the first half cycle of the alternating voltage, corresponding onesof the LED devices are connected in series to one another to form thefirst current loop along (A1, A1′)-C1-(B2, B2′)-C2-A3-(C3,C3′)-B4-C4-A5. In the second half cycle of the alternating voltage,other corresponding LED devices are connected in series to one anotherto have the second current loop alongB1-C1--A2-C2--B3-(C3,C3′)-A4-C4-C4′-B5. However, devices in parenthesesare connected in parallel with one another.

An increase in the LED devices located on the middle branches also leadsto an increase in the number of devices driven in a bi-direction,thereby enhancing the utilization efficiency of the LEDs. However, onlyan increase in the number of the LED devices located on the middlebranches results in an increase in a reverse directional voltage appliedto the LED devices belonging to the first and second branches.Therefore, in a case where the LED devices are of an identical size, thenumber of the LEDs located on the middle branches may be two or three.

In a specific embodiment of the present invention, a plurality of theladder network circuits are provided. Here, a second junction point ofone of the ladder network circuits maybe connected in series with afirst junction point of another ladder network circuit. This embodimentis shown in FIG. 3.

Referring to FIG. 3, an LED driving circuit is structured such that twoladder network circuits are connected in series to each other. That is,a second junction point b1 of a first ladder network circuit isconnected to a first junction point a2 of a second ladder network. Also,a first junction point a1 of the first ladder network circuit and asecond junction point of the second ladder network are connected to anAC power terminal. Moreover, in the present embodiment, two LED devicesare disposed on a first branch, a second branch and middle branches,respectively to be connected in series to each other.

In the LED driving circuit of FIG. 3, in a first half cycle of analternating voltage, corresponding ones of the LED devices are connectedin series to one another to form a first current loop alongA1-A1′-C1-C1′-B2-B2′-C2-C2′-A3-A3′ (hereinafter, first ladder networkcircuit)-B4-B4′-C3-C3′-A5-A5′-C4-C4′-B6-B6′ (hereinafter, second laddernetwork circuit). In a second half cycle of the alternating voltage,other corresponding LED devices are connected in series to one anotherto have a second current loop along B1-B1′-C1-C1′-A2-A2′-C2-C2′-B3-B3′(hereinafter, first ladder networkcircuit)-A4-A4′-C3-C3′-B5-B5′-C4-C4′-A6-A6′ (hereinafter, second laddernetwork circuit).

In the LED driving circuit of the present embodiment, eight LED devicesC1, C1′, C2, C2′, C3, C3′, C4, and C4′ belong to the middle branches.That is, two times greater number of the LED devicesC1,C1′,C2,C2′,C3,C3′,C4, and C4 are commonly involved in the first andsecond current loops to ensure continuous operation over an entire cycleof the alternating voltage compared with the LED driving circuit of FIG.2A.

As described above, the LEDs maybe arranged in various configurations toallow for AC driving of the ladder network structure according to thepresent embodiment.

According to an exemplary embodiment of the invention, there may beprovided an LED array device having a plurality of LED devices includingLED driving circuits of various ladder network structures.

That is, in the LED array apparatus of the present invention, K numberof first LED devices are connected in a row by n number of first middlejunction points between first and second junction points. Also, thefirst middle junction points each have electrodes of identical polarityconnected thereto.

The electrodes connected to the first middle junction points havepolarities arranged alternately from the first junction point, startingwith a first polarity. Here, K is an integer satisfying K≧3, and n is aninteger satisfying n≧2. L number of second LED devices are connected ina row by n number of second middle junction points between the first andsecond junction points. Also, the second middle junction points eachhave electrodes of identical polarity connected thereto. The electrodesconnected to the second middle junction points have polarities arrangedalternately from the first junction point, starting with a secondpolarity. Here, L is an integer satisfying L≧3. Thus, the electrodesconnected to the first and second middle junction points have oppositepolarities to each other.

Furthermore, M number of third LED devices disposed on the middlebranches of the circuit are connected between the first and secondmiddle junction points each having an identical sequence from the firstjunction point. The third light emitting diodes each have an electrodeconnected to the first and second middle junction points of an identicalm sequence to have an opposite polarity to a corresponding one of theelectrodes of the first and second LEDs. Here, M is an integersatisfying M≧m and m is a positive integer defining a sequence of thefirst and second middle junction points from the first junction point.

Each of the first LED devices and each of the second LED devices may belocated between the middle junction points, respectively. Similarly,each of the third LED devices may be located between the first andsecond middle junction points, respectively.

Optionally, the plurality of third LED devices may be connected betweenthe first and second middle junction points. Here, the third LED devicesmay be connected in series or in parallel to one another (see FIG. 2B or2C).

To explain effects of less LED numbers in the ladder network LED drivingcircuit of the present embodiment, a difference has been comparedbetween the numbers of LEDs required in the ladder network LED drivingcircuit of the present embodiment and the numbers of LEDs required inthe conventional AC driving LED circuit, e.g., bi-polar circuit andbridge network circuit in order to satisfy specific output conditionsusing identical LED devices.

First, FIGS. 4A to 4C illustrate LED driving circuits according to aconventional example and according to an exemplary embodiment of theinvention.

The LED driving circuit shown in FIG. 4A is a reverse-parallel circuitstructure for general AC driving in which LED devices 30A and 30Barranged in reverse parallel are connected in series to each other todefine a plurality of stacks S. As shown in Table 1, despite an overallincrease in the number of stacks, a ratio of the number of continuouslydriven LEDs to the number of employed LEDs, i.e., utilization efficiencyof LEDs is 50%.

The LED driving circuit shown in FIG. 4B is a bridge circuit structurein which each LED device is located in each branch. One stack includesall five LED devices 40A, 40B, 40C, 40D, and 40E and the plurality ofstacks maybe formed to ensure desired output. As noted in Table 1, thisbridge network LED circuit has a utilization efficiency of 60%regardless of the number of the stacks. This is attainable becauseunlike the reverse-parallel arrangement of FIG. 4A, the LED devices 40Elocated in the middle branch can be driven continuously in abi-direction.

In the same manner as FIG. 2A, the ladder network LED driving circuitshown in FIG. 4C includes a total of eight LEDs to define two stacks.Five LEDs are continuously driven to ensure a high utilizationefficiency of 62.5%. Also, as noted in Table 1, the ladder network LEDdriving circuit is configured such that with a greater number of thestacks, a greater number of LEDs are driven in a bi-direction. Thisaccordingly leads to a gradual increase in utilization efficiency of theLEDs.

TABLE 1 Reverse parallel network Bridge network Ladder network NumberNumber Number of of of Number Number turns-on Number turns- Numberturns- of of in bi- Efficiency of on in Efficiency of on in Efficiencystacks V_(f) LEDs direction (%) V_(f) LEDs bi-direction (%) V_(f) LEDsbi-direction (%) 1 ΔV_(f) 2 0 50  3 · ΔV_(f) 5 1 60  5 · ΔV_(f) 8 2 62.52 2 · ΔV_(f) 4 0 50  6 · ΔV_(f) 10 2 60  7 · ΔV_(f) 11 3 63.6 3 3 ·ΔV_(f) 6 0 50  9 · ΔV_(f) 15 3 60  9 · ΔV_(f) 14 4 64.3 4 5 · ΔV_(f) 8 050 12 · ΔV_(f) 20 4 60 11 · ΔV_(f) 17 5 64.7 5 5 · ΔV_(f) 10 0 50 15 ·ΔV_(f) 25 5 60 13 · ΔV_(f) 20 6 65 6 6 · ΔV_(f) 12 0 50 18 · ΔV_(f) 30 660 15 · ΔV_(f) 23 7 65.2 7 7 · ΔV_(f) 14 0 50 21 · ΔV_(f) 35 7 60 17 ·ΔV_(f) 26 8 65.4 8 8 · ΔV_(f) 16 0 50 24 · ΔV_(f) 40 8 60 19 · ΔV_(f) 299 65.5 9 9 · ΔV_(f) 18 0 50 27 · ΔV_(f) 45 9 60 21 · ΔV_(f) 32 10 65.610 10 · ΔV_(f ) 20 0 50 30 · ΔV_(f) 50 10 60 23 · ΔV_(f) 35 11 65.7 2121 · ΔV_(f ) 42 0 50 63 · ΔV_(f) 105 21 60 45 · ΔV_(f) 68 22 66.2 30 30· ΔV_(f ) 60 0 50 90 · ΔV_(f) 150 30 60 63 · ΔV_(f) 95 31 66.3 63 63 ·ΔV_(f ) 126 0 50 — — — — — — — —

Therefore, in case of requiring an output of nine LED devices, thereverse-parallel LED circuit shown in FIG. 4A requires a total ofeighteen LED devices and the bridge network LED circuit requires a totalof fifteen LED devices to define three stacks. Meanwhile, in the laddernetwork LED circuit according to the present invention, a total fourteenLEDs are connected to define three stacks, thereby providing desiredlight amount (nine LED devices). This leads to a considerable decreasein the number of LED devices employed over the bridge LED circuit.

This improvement is further achieved in the circuit with a biggeroutput. That is, in case of requiring an output of sixty three LEDdevices, the reverse-parallel circuit and the bridge circuit requiresone hundred twenty six and one hundred five LED devices, respectively toenable AC driving circuit. However, the ladder network LED circuitrequires only ninety five LED devices, thereby reducing the number ofthe LED devices by 31 and 10, respectively, over the conventionalcircuit.

This is because in the bridge LED circuit, at least two LED devices arelocated in a current loop between the LEDs commonly driven in abi-direction. Meanwhile, in the ladder network, at least one LED deviceis required between the LED devices commonly utilized. That is, theladder network circuit requires a less number of LEDs between the LEDscommonly utilized in a bi-direction than the bridge network circuit.This allows the ladder network to commonly utilize a greater overallnumber of LEDs in a bi-direction than the bridge structure.

FIGS. 5A to 5B illustrate LED driving circuits according to anotherconventional example and according to another exemplary embodiment ofthe invention.

The driving circuits of FIGS. 5A and 5B are similar to those of FIGS. 4Band 4C but configured such that two LED devices are located in each ofmiddle branches to be connected in series to each other. That is, thenumber of continuously driven LED devices is increased to an equal levelin each of stacks. The ladder network LED driving circuit shown in FIG.5B will be understood with reference to the embodiment shown in FIG. 2B.

TABLE 2 Reverse parallel network Bridge network Ladder network NumberNumber Number of of of Number Number turns-on Number turns- Numberturns- of of in bi- Efficiency of on in Efficiency of on in Efficiencystacks V_(f) LEDs direction (%) V_(f) LEDs bi-direction (%) V_(f) LEDsbi-direction (%) 1 ΔV_(f) 2 0 50  4 · ΔV_(f) 6 2 66.7  7 · ΔV_(f) 10 470 2 2 · ΔV_(f) 4 0 50  8 · ΔV_(f) 12 4 66.7 10 · ΔV_(f) 14 6 71.4 3 3 ·ΔV_(f) 6 0 50 12 · ΔV_(f) 18 6 66.7 13 · ΔV_(f) 18 8 72 4 5 · ΔV_(f) 8 050 16 · ΔV_(f) 24 8 66.7 16 · ΔV_(f) 22 10 72.7 5 5 · ΔV_(f) 10 0 50 20· ΔV_(f) 30 10 66.7 19 · ΔV_(f) 26 12 73.1 6 6 · ΔV_(f) 12 0 50 24 ·ΔV_(f) 36 12 66.7 22 · ΔV_(f) 30 14 73.3 7 7 · ΔV_(f) 14 0 50 28 ·ΔV_(f) 42 14 66.7 25 · ΔV_(f) 34 16 73.5 8 8 · ΔV_(f) 16 0 50 32 ·ΔV_(f) 48 16 66.7 28 · ΔV_(f) 38 18 73.7 9 9 · ΔV_(f) 18 0 50 36 ·ΔV_(f) 54 18 66.7 31 · ΔV_(f) 42 20 73.8 10 10 · ΔV_(f ) 20 0 50 40 ·ΔV_(f) 60 20 66.7 34 · ΔV_(f) 46 22 73.9 13 13 · ΔV_(f ) 26 0 50 52 ·ΔV_(f) 78 26 66.7 43 · ΔV_(f) 58 28 74 16 16 · ΔV_(f ) 32 0 50 64 ·ΔV_(f) 96 32 66.7 52 · ΔV_(f) 70 34 74.3 52 52 · ΔV_(f ) 104 0 50 — — —— — — — —

Therefore, in case of requiring an output of sixteen LED devices, thereverse-parallel LED circuit shown in FIG. 4A requires a total thirtytwo LED devices, and the bridge network LED circuit shown in FIG. 5Arequires a total twenty four LED devices to define four stacks.Meanwhile, in the ladder network LED circuit of the present invention, atotal twenty two LED devices are required to provide desired lightamount (sixteen LED devices). This leads to considerable reduction inthe number of the LED devices employed over the bridge LED circuit.

This improvement is further achieved in the circuit with a biggeroutput. That is, in case of requiring an output of fifty two LEDdevices, the reverse-parallel circuit and the bridge circuit require onehundred four and seventy eight LED devices, respectively to enable ACdriving circuit. However, the ladder network LED circuit requires onlyseventy LED devices, thereby reducing the number of the LED devices by34 and 8, respectively over the conventional circuit.

As described above, the ladder network LED driving circuit requires amuch smaller number of LED devices fore AC driving to achieve identicaloutput than the conventional reverse-parallel structure and the bridgestructure as well.

As set forth above, according to exemplary embodiments of the invention,a novel LED driving circuit adopts a ladder network principle toincrease a ratio of the number of continuously driven LED devices to atotal number of employed

LED devices. This leads to relative decrease in the number of LEDdevices employed to achieve equal output and subsequently improvement inoverall efficiency of the circuit. Also, an LED array apparatusincluding the LED driving circuit can be configured.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. An AC driven light emitting device comprising: a plurality of firstLEDs, each having a first polarity and a second polarity, linearlyconnected by (n) number of first middle junction points, in which thesame polarities of adjacent first LEDs are connected each other, where ndenotes an integer satisfying n>2; a plurality of second LEDs, eachhaving a first polarity and a second polarity, linearly connected by (n)number of second middle junction points, in which the same polarities ofadjacent second LEDs are connected each other; a first junction point inwhich different polarities of a first LED of the plurality of first LEDsand a first LED of the plurality of second LEDs are connected eachother; a second junction point in which different polarities of a lastLED of the plurality of first LEDs and a last LED of the plurality ofsecond LEDs are connected each other; and a plurality of third LEDs,each having a first polarity and a second polarity, connected betweenthe first and second middle junction points in the same order such thatthe associated polarities of the plurality of third LEDs are connectedto the first and second middle junction points having the differentpolarities, wherein upon applying an alternating voltage between thefirst and second junction points, the first and second LEDs are drivenin a half cycle of the alternating voltage and the third LEDs are drivenin an entire cycle of the alternating voltage.
 2. The AC driven lightemitting device of claim 1, wherein the first and second LEDs arearranged such that single LED is disposed between the adjacent middlejunction points.
 3. The AC driven light emitting device of claim 1,wherein the first and second LEDs are arranged such that at least twoLEDs are connected in series between the adjacent middle junctionpoints.
 4. The AC driven light emitting device of claim 1, wherein thethird LEDs are arranged such that at least two LEDs are connected inseries between the first and second middle junction points.
 5. Alighting apparatus comprising the AC driven light emitting device ofclaim 1.